A multipole rod assembly is a device operated to control the motion of ions by generating a radio-frequency (RF) or a composite RF/direct-current (DC) electrical field in an interior region or volume of the device into which ions may be transmitted. The multipole rod assembly includes a set of rods, (i.e., rod-shaped electrodes) that extend in an axial direction. The rods are each positioned at some radial distance from a central, longitudinal axis of symmetry about which the rods are arranged. In the transverse plane orthogonal to the longitudinal axis, the rods are circumferentially spaced from each other. The rods thus coaxially surround and define the interior volume in which ions may be introduced, or in some case produced if an appropriate ionizing device is provided in conjunction with the multipole rod assembly. With this axial geometry, the multipole rod assembly may be referred to as a “linear” or “two-dimensional” multipole rod assembly. Typically, the rods are arranged in parallel with the common longitudinal axis, although in some applications may converge toward or diverge away from the longitudinal axis in the direction from the entrance to the exit of the interior volume. Multipole rod assemblies typically include an even number of rods. Common examples include a quadrupole arrangement (four rods), a hexapole arrangement (six rods), and an octopole arrangement (eight rods), although higher-order multipole arrangements containing more rods are possible.
Electronics provided with the multipole rod assembly include one or more voltage sources communicating with individual rods and/or sets of electrically interconnected rods. The voltages applied to and/or between the rods are configured to generate at least a two-dimensional, time-varying RF electric field in the interior volume. This RF electric field generally extends along the entire length of the rods and thus along the entire length of the interior volume surrounded by the rods. Accordingly, an ion generally at any location in the interior volume will be exposed to and influenced by the RF electric field. The RF electric field is configured (i.e., as to spatial orientation and energy distribution) to confine the motions of ions in the interior volume to the vicinity of the longitudinal axis. That is, the RF electric field focuses the ions into an ion beam at the longitudinal axis. The operating parameters of the RF field (voltage amplitude and frequency) determine whether the motion or trajectory of an ion of a given mass-to-charge ratio (or m/z ratio, or more simply “mass”) is stable or unstable in the RF electric field. A stable ion can travel through the full length of the multipole rod assembly as part of the focused beam and exit the multipole rod assembly. An unstable ion will deviate from the focused beam without being sufficiently repelled by the RF electric field back toward the center (longitudinal axis) of the interior volume, and consequently will impact a rod and be neutralized thereby or escape the interior volume through the space between a pair of adjacent rods. A multipole rod assembly operated as an RF-only ion guide can potentially transmit a broad range of ions (ions having a broad range of m/z ratios).
In the special case of a quadrupole rod assembly, DC voltages can be superposed on the RF voltages applied to the rods to generate a composite RF/DC electric field in the interior volume. The composite RF/DC electric field, defined by well-known mathematical relations in the case of a quadrupole arrangement, not only focuses the ions as an ion beam on the longitudinal axis but also imposes an m/z ratio passband on the transmission of ions through the quadrupole rod assembly. The limits or end points of the m/z ratio passband (the low-mass cutoff point and high-mass cutoff point), and the width of the m/z ratio passband between the low-mass and high-mass cutoff points, are dictated by the operating parameters of the composite RF/DC electric field (RF voltage amplitude and frequency, and DC voltage magnitude). For example, the m/z ratio passband may be configured to pass only ions having a particular m/z ratio (e.g., m/z=105), or ions falling within a narrow range of m/z ratios (e.g., m/z=100 to m/z=110). Ions transmitted into the quadrupole rod assembly having m/z ratios falling in the m/z ratio passband will have stable trajectories and thus a high probability of passing through the full length of the multipole rod assembly and exiting therefrom. On the other hand, ions transmitted into the quadrupole rod assembly having m/z ratios outside of the /z ratio passband will have unstable trajectories and thus will not successfully traverse the full length of the multipole rod assembly and exit therefrom, i.e., such ions will be rejected by the quadrupole rod assembly. Moreover, as the stability of an ion depends on its m/z ratio as well as the operating parameters of the composite RF/DC electric field, one or more of the operating parameters can varied over time, which has the effect of scanning ion masses in succession. For example, the ions may be scanned such that ions of m/z=100 are transmitted (selected) while all other ions are rejected, then ions of m/z=101 are transmitted while all other ions are rejected, then ions of m/z=103 are transmitted while all other ions are rejected, and so on. A quadrupole rod assembly generating such a composite RF/DC electric field may thus be utilized as a mass-selective device such as a mass filter or mass analyzer.
One common application for such a quadrupole rod assembly is a mass spectrometry (MS) system having a “triple-quad” or “QqQ” configuration. The triple-quad MS system includes a first-stage mass filter or mass analyzer, followed by a collision cell, and in turn followed by a second-stage mass filter or mass analyzer. A sample of material to be analyzed is ionized, and the resulting analyte ions are transmitted into the first-stage mass filter or mass analyzer as “precursor” ions. Typically, the first-stage mass filter or mass analyzer selects precursor ions of one selected m/z ratio for further transmission into the collision cell. The collision cell fragments these precursor ions into product (or fragment) ions, which have a range of m/z ratios smaller than the m/z ratio of the precursor ions, and transmits these product ions to the second-stage mass filter or mass analyzer. The second-stage mass filter or mass analyzer then transmits the product ions to an ion detector, often in accordance with a scanning function. The ion detector outputs electrical signals to electronics for signal processing as needed to generate a mass spectrum representative of characteristics of the sample. In such an application, a quadrupole rod assembly is often employed as the first-stage mass filter or mass analyzer and/or the second-stage mass filter or mass analyzer. A quadrupole rod assembly may also be employed as an RF-only ion guide in the collision cell (hence the traditional name “triple-quad”), although more often the collision cell utilizes a multipole rod assembly of higher order (e.g., a hexapole or octopole).
From the foregoing, it is evident that to ensure a quadrupole rod assembly processes ions in an accurate, predictable, and repeatable manner, the electric field(s) generated and maintained by the quadrupole rod assembly should be as pure and uniform as possible over the entire axial length of the quadrupole rod assembly. This means that any unintended perturbations or defects in the electric field(s), such as may be manifested by fringing effects, non-linearities, and localized higher-order fields, should be minimized as much as possible. The physical geometry of the rods, particularly their surfaces that face the interior volume and to which the ions are thus exposed, and the relative positions of the rods, have a direct effect on the purity and uniformity of the electric field(s). Hence, it is critical that a quadrupole rod assembly be fabricated and assembled in a precise manner, with minimal tolerances. The surface of each rod facing the interior volume should be accurately shaped. The shape of each rod should be uniform along the entire axial length of the rod, and should be the same as the shapes of the other rods as much as possible (i.e., with minimal tolerances). The distance of each rod from the other rods should be uniform along the entire axial length of each rod as much as possible (i.e., with minimal tolerances).
Moreover, the foregoing attributes must be as insensitive to temperature as possible, i.e., thermal expansion should be minimized as much as possible. The maximization of temperature insensitivity in quadrupole rod assemblies is an ongoing challenge. The fixing of the positions of the rods in space and the mounting of the rods in an instrument require the use of electrically insulating components and mounting hardware. The materials utilized for the electrically conductive rods and the materials utilized for the electrically insulating components are necessarily different and thus have different coefficients of thermal expansion. The material composition of the mounting hardware is also different from the rods and/or the electrically insulating components. Consequently, as electrical power is applied to the rods during operation, the rods, electrically insulating components, and mounting hardware are heated and undergo thermal expansion to different degrees, which can lead to distortions in the geometry and position of the rods and consequently impurities and non-uniformities in the electric field(s).
Generally, the foregoing considerations apply to higher-order multipole rod assemblies as well. However, the desired level of precision in the positioning of the rods and the temperature insensitivity of the rod assemblies can be less rigorous, as higher-order multipole rod assemblies are not utilized to select or scan ions on the basis of m/z ratios, and thus a greater degree of field impurity and non-uniformity is acceptable in comparison to quadrupole rod assemblies utilized as mass-selective devices.
In view of the foregoing, there is an ongoing need for providing quadrupole rod assemblies, and by extension higher-order multipole rod assemblies, with improved geometric and positional precision and temperature insensitivity.